In a spark plug used for igniting an internal-combustion engine, such as a gasoline engine for an automobile, the construction of an engine head has been sophisticated according to an advance of an engine in the recent years. Further, a miniaturization of a spark plug and a reduction in the diameter thereof are in demand due to a reduction in a mounting space of a spark plug. In order to miniaturize and reduce the diameter of a spark plug, the diameter of a metal shell having a mounting portion being fixed to the engine head is reduced, and the thickness of an insulator held inside the metal shell needs to be thinned as well as the diameter of the insulator reduced.
However, when the insulator is made thin and the diameter thereof reduced, a tendency of causing a dielectric breakdown generated between the metal shell enclosing the circumference of the insulator and a center electrode inserted in an axial hole through penetrating the insulator may increase. Further, it is difficult to secure the withstand voltage. In order to meet these conflicting demands, the withstand voltage of the insulator is desired to be high.
As a concrete means to solve the above-mentioned problems, for example, Japanese Patent Application Laid-Open (kokai) No. H9-272273 discloses an alumina ceramic having a pore rate of 0.5% or less. Further, Japanese Patent Application Laid-Open (kokai) No. H11-45143 discloses an alumina-based sintered body having pores which are exposed on a predetermined mirror-finishing face, occupying 4% orless of the mirror-finishing face and having a maximum length of 15 μm or less.